The comedian Jon Richardson used to do a gag about obsessive-compulsive disorder. "I never say I've got OCD," it went, "because it's got the word 'disorder' in it and it seems untidy." Well, perhaps you had to be there – the past tense being particularly relevent in this case because Richardson doesn't tell this joke anymore. Jon Richardson: a Little Bit OCD, his contribution to Channel 4's season of programmes about mental health, offered an explanation of why the line had been dropped.

Richardson has used his own obsessions as raw material for his comedy and he offered up examples of them almost immediately, marking down a comedy venue because of its regrettable lack of symmetry and staring daggers at an untidy wall decoration in his dressing room. He has self-imposed rules about how he walks down the pavement, smooths out the butter to a level surface every time he uses it and has very fixed ideas about how to fill a cutlery drawer.

At one point, he met up with the comedian Russell Howard and some other friends to discuss the "spoon wars" that had broken out when they shared a flat. Thinking his obsession with spoon-related tidiness rather comical they had deliberately tweaked his anxieties by leaving spoons everywhere. Richardson shocked them by revealing that this had made him so anxious and upset that he'd slept in his car rather than share living space with rogue spoons.

Worried about whether this constituted a personal quirk or the beginnings of full-blown OCD, Richardson visited others with the condition and psychiatric professionals to talk about the illness. And it turned out that interviewing people with OCD involves quite a lot of protocol. Gemma, who is virtually housebound because of her compulsion with cleaning her flat, would only allow him on to her external balcony, and then only if he wiped his feet according to her careful specifications.

Joyce, who can happily care for chickens and goats outside her house, but operates a rigid decontamination procedure inside it, let him in only on conditions that he touched nothing (except paper, which she doesn't worry about because it can't be washed anyway). She also shared the terrible story of her son, who killed himself with a yew-leaf smoothie when he could see no way out of the prison of his own obsessions.

Richardson's personal discoveries were less disturbing. He wasn't exactly comfortable when he was asked to try out a standard desensitisation treatment – which consisted of wiping his hands on a toilet seat and then all over his head – but then who would be? And an expert in the condition noted that his own taboos didn't seem to involve any large-scale disruption in his life. "I feel I have obsessive-compulsive order," Richardson concluded, reasonably happy that, for the moment at least, he controls it rather than it controlling him. But the glimpse of lives in which the balance of power was reversed left you in no doubt that in the worst cases it's no laughing matter.

The last episode of Twenty Twelve (and it pains me to even write those words) went out with a Sopranos finish, cutting to black at a critical moment as Ian worked himself up to say something of large significance to Sally. I still can't work out whether this was a cruel withholding of a consummation we've been longing for, or a wise decision to let us fill in the blank ourselves. But I can't hold it against the series, which has ridden the razor's edge between straight transcription and satirical exaggeration with near-perfect balance. Highlights this week included the discovery that the opening fireworks might trigger the army's ground-to-air missiles and last-minute glitches with a cultural commission involving mass bell ringing (that one was transcription surely). Lowlights next week include the fact that it isn't on any more and we have to swallow our Olympibollocks neat.